a) Field of the Invention
The invention relates to a process and a plant for the separation of air by cryogenic distillation. In particular, it relates to a process for producing pure oxygen using a mixing column and, possibly, for producing argon using an argon column.
b) Description of Prior Art
In EP-A-0229803, the mixing column is fed at the bottom with vaporized rich liquid coming from the head condenser of the argon column.
EP-A-0269342 relates to the case in which the argon column is thermally coupled to a mixing column so that the overhead gas of the argon column warms the mixing column.
U.S. Pat. No. 5,551,258 describes a process in which a mixing column is fed at the top with a liquid containing 55 vol % oxygen, the overhead gas of the mixing column then serving to warm the bottom condenser of the low-pressure column.
One object of the present invention is to increase the amount of pure gaseous oxygen (containing more than 99.5 mol % oxygen) which can be produced by a double air separation column.
One object of the invention is an air separation plant for producing an oxygen-rich fluid by cryogenic distillation, comprising:
a double column comprising at least one medium-pressure column and a low-pressure column which includes a reboiler for vaporizing the liquid in the bottom of the low-pressure column;
a mixing column;
means for sending cooled and compressed air to the medium-pressure column;
means for sending an oxygen-enriched liquid from the medium-pressure column to an injection point of the low-pressure column and means for sending a nitrogen-enriched liquid from the medium-pressure column to the low-pressure column;
means for sending gas to the bottom of the mixing column;
means for sending a second oxygen-enriched liquid from the low-pressure column to the top of the mixing column, this second liquid being less volatile than the gas feeding the bottom of the mixing column;
means for withdrawing an oxygen-rich fluid from the low-pressure column;
means for sending at least one portion of a gas, possibly the overhead gas, from the mixing column to passages for warming the reboiler,
characterized in that it includes means for withdrawing the second liquid from the low-pressure column at a level at least 5 theoretical trays below the injection point of the low-pressure column, or possibly at least 10 theoretical trays below the injection point of the low-pressure column and/or about a third of the way up the low pressure column.
The apparatus may include an argon column or simply a vaporizer for vaporizing liquid form the bottom of the medium pressure column by heat exchange with a gas from the low pressure column.
Preferably it comprises means for withdrawing the second liquid at a level between the bottom of the low pressure column and the point of removal of the feed to the argon column or the point of removal of the gas sent from the low pressure column to the rich liquid vaporizer.
In this case, the mixing column may be fed at the bottom with any gas more volatile than the liquid oxygen sent to the top of the column.
Another object of the invention is an air separation plant for producing an oxygen-rich fluid and possibly an argon enriched fluid by cryogenic distillation, comprising:
a double column comprising a medium-pressure column and a low-pressure column which includes a reboiler for vaporizing the liquid in the bottom of the low-pressure column;
a condenser, possibly at the top of an argon column;
a mixing column;
means for sending cooled and compressed air at least to the medium-pressure column;
means for sending a first oxygen-enriched liquid from the medium-pressure column to the condenser;
means for withdrawing an argon-enriched gas from a first level of the low-pressure column and means for sending it to the condenser or to the argon column;
means for sending an oxygen-enriched liquid from the medium-pressure column to at least one injection point of the low-pressure column and a nitrogen-enriched liquid from the medium-pressure column to the low-pressure column;
means for vaporizing oxygen-enriched liquid in the condenser and means for sending at least one portion of the vapour and/or air to the bottom of the mixing column;
means for sending a second oxygen-enriched liquid from the low-pressure column to the top of the mixing column, this second liquid being less volatile than the gas feeding the bottom of the mixing column;
possibly means for withdrawing an argon-enriched fluid at the top of the argon column;
means for withdrawing an oxygen-rich fluid from the low-pressure column and
means for sending at least one portion of a gas, possibly the overhead gas, from the mixing column to passages for warming the reboiler.
Preferably the apparatus comprises means for withdrawing the second liquid is withdrawn from the low-pressure column at a level at least 5 theoretical trays below the point of injection or the lowermost point of injection at a level (still more preferably at least 10 theoretical trays below the point of injection or the lowermost point of injection) and/or about a third of the way up the low-pressure column.
According to other optional aspects:
the overhead gas of the mixing column at least partially condenses in the reboiler and at least part of the condensate is sent to the low pressure column, preferbaly at a level above the first level;
there are means for sending the liquid in the bottom of the argon column to the top of the mixing column;
there are means for sending a bottom liquid and/or an intermediate liquid from the mixing column to the double column;
there are at least 80, preferably at least 90, theoretical trays in the low-pressure column;
there are means for sending air or a gas from the medium-pressure column to other passages for warming the reboiler;
the apparatus includes a blowing turbine for sending air to the low pressure column or the mixing column;
the apparatus comprises means for withdrawing gaseous nitrogen from the medium-pressure column as a product.
Another object of the invention is a process for the separation of air by cryogenic distillation for producing oxygen with a double column comprising:
a medium-pressure column; and
a low-pressure column which includes a reboiler for vaporizing the liquid in the bottom of the low-pressure column,
comprising the steps of:
sending cooled and compressed air to the medium-pressure column;
sending an oxygen-enriched liquid and a nitrogen-enriched liquid from the medium-pressure column to the low-pressure column;
sending gas to the bottom of the mixing column;
sending a second oxygen-enriched liquid from the low-pressure column to the top of the mixing column, this second liquid being less volatile than the gas feeding the bottom of the mixing column;
withdrawing an oxygen-rich fluid from the low-pressure column;
sending at least one portion of a gas, possibly the overhead gas, from the mixing column to passages for warming the reboiler,
characterized in that the second liquid contains less than 5 mol % nitrogen and/or the gas sent from the mixing column to the warming passages contains less than 15 mol % nitrogen.
Another object of the invention is a process for the separation of air by cryogenic distillation for producing oxygen and possibly argon with a double column comprising:
a medium-pressure column;
a low-pressure column which includes a reboiler for vaporizing the liquid in the bottom of the low-pressure column; and
a condenser, optionally at the top of an argon column, and a mixing column,
comprising the steps of:
sending cooled and compressed air to at least the medium-pressure column;
sending a first oxygen-enriched liquid from the medium-pressure column to the head condenser;
withdrawing an argon-enriched gas from a first level of the low-pressure column and sending it to the condenser or the argon column;
sending an oxygen enriched liquid and a nitrogen-enriched liquid from the medium-pressure column to the low-pressure column;
at least partially vaporizing oxygen-enriched liquid in the condenser and sending at least one portion of the vapour and/or air to the bottom of the mixing column;
sending a second oxygen-enriched liquid from the low-pressure column to the top of the mixing column, this second liquid being less volatile than the gas feeding the bottom of the mixing column;
possibly withdrawing an argon-enriched liquid from the top of the argon column;
withdrawing an oxygen-rich fluid from the low-pressure column;
sending at least one portion of a gas, possibly the overhead gas, from the mixing column to passages for warming the reboiler,
characterized in that the second liquid contains less than 5 mol. % nitrogen and/or the gas sent from the mixing column to the warming passages contains less than 15 mol. % nitrogen.
According to other optional aspects:
the overhead gas of the mixing column at least partially condenses in the reboiler and the condensate is sent to the low pressure column, possibly at a level above the first level;
at least part of the liquid in the bottom of the argon column is sent to the top of the mixing column;
a bottom liquid and/or an intermediate liquid are sent from the mixing column to the double column;
a gas from the medium-pressure column or air at least partially condenses in other passages for warming the reboiler;
the overhead gas of the mixing column comprises 3 to 5 mol % nitrogen;
the overhead gas of the mixing column comprises at least 93 mol. %, possibly at least 95 mol. % oxygen;
the liquid sent to the top of the mixing column contains at least 98 mol % oxygen;
the pressure of the mixing column is at between 0,5 and 1 bar above the pressure of the low pressure column.
It will be understood that the overhead gas of the mixing column may be withdrawn from the top of the mixing column or at most five theoretical trays below the top of the mixing column.